It seems that because the reverse DNS lookup (PTR) fails, both anti-spam measurements (PTR check + DNS Blacklist check) are being skipped.

I you lookup the PTR for 116.104.173.166 on various online tools, you'll get or timeouts or 'no results found'. I expect the timeout is the issue I'm facing. But effectively this means that spam which is sent from an IP that has a faulty (or slowish) PTR DNS configuration ensures the spam being delivered on my MailEnable SMTP agent right (as further check are being skipped)?

How to counter this spam giving the above? And what is your read, a bug or edge case within MailEnable, or misconfiguration from my side?